1. Field of the Invention
The present invention relates to a structure for an ink jet head.
2. Description of the Prior Art
There is in the prior art an ink jet head of the so-called "on-demand" type for printing by ejecting ink from a nozzle. This ink jet head is formed with a common ink chamber, a plurality of ink passages to be supplied with ink from the ink chamber, and a nozzle at the leading end of each ink passage. This ink passage is formed with a pressure chamber which is equipped with ink ejecting means such as a piezoelectric element for imparting flying force to the ink.
Most nozzles of the prior art are shaped to have their sectional areas gradually decreased from their entrances (located at the side of the ink chamber) toward their exits (at the side of a printing medium), as disclosed in Japanese Patent Laid-Open No. 178768/1982, or to have their sectional areas once decreased and restored to a constant value, as disclosed in Japanese Patent Publication No. 44549/1988. In either construction, the nozzle has the minimum sectional area at its exit and is opened in the head end face.
On the other hand, the nozzle having a construction, in which it is arranged on a common plane with the pressure chamber, is formed by etching or injection-molding a material of glass, metal, semiconductor or plastics to form a groove, subsequently by bonding the grooved material and a substrate member, and finally by cutting or grinding the head end face to a predetermined nozzle length. On the other hand, the construction having its pressure chamber and nozzle facing each other is made by adhering a substrate having ink ejecting means and a passage substrate.
In case the ink jet head is to be constructed by the aforementioned methods, the nozzle is liable to have its exit periphery broken or deformed at the cutting or grinding step.
If a nozzle a has its exit a1 broken at a2, as shown at the righthand side of FIG. 8, for example, ink droplets b have their surface tensions unbalanced when they leave the head end face, so that they go out of their intrinsic orbit c to another orbit d which is offset to the notch a2. Thus, correct printing cannot be achieved.
In another case, too, in which a nozzle e is constructed to have a normal exit e1, as shown at the lefthand side of FIG. 8, the ink will ooze to the nozzle periphery at the head end face, as the drive frequency rises. If the ooze is deviated, the ink droplets b will also brought out of the intrinsic orbit c to another orbit f by the tension to the oozing side, so that correct printing can neither be achieved.
There are still many problems in which the nozzle exit has its length changed with the resultant change in the ejection characteristics of the ink droplets by the errors in the cutting or grinding step. It is known that especially the portion having the smaller effective area will exert the more influences upon the ejection characteristics of the ink droplets. The above-specified constructions of the prior art are liable to cause errors in the nozzle exit having the minimum effective area so that they can hardly achieve the desired ejection characteristics.